Switching System and Method

ABSTRACT

The invention relates to a Radio Frequency System and method. A Radio Frequency (RF) system comprising a RF switch comprising a plurality of transistor switching elements implemented on Silicon on Insulator (SOI) for switching at least one or more RF signals and said SOI comprises a bulk substrate region and a buried oxide region. At least one filter is adapted to isolate the RF signal from the substrate and/or other high frequency signals or control signals present in the RF system. There is also provided a coupling capacitor adapted to cooperate with the filter to improve linearity of the transistor switch elements.

FIELD OF THE INVENTION

The present invention generally relates to the field of semiconductorsand RF systems. In particular, the invention relates to RF switches andto switching radio frequency (RF) signals within an integrated circuit.

BACKGROUND TO THE INVENTION

RF switches are a key building block in wireless systems and find manyuses in applications such as mobile phones and wireless LANS. The act ofswitching radio frequency signals in an integrated circuit is carriedout by an RF switch circuit. Many technologies exist for the design ofintegrated RF switch circuits.

Generally an RF switch system does not consist of the RF switchingcircuit alone and typically must be controlled using digital logiccircuitry. In addition both negative and positive voltage levels arerequired to optimise the performance of the switch. Therefore, anegative voltage generator may also be required as part of the switchsystem. A switch can have a number of switching elements, although itcan be comprised of any number of switching elements, which control theflow of RF power between different points in the particular application.Often MOSFET switches are used as the switching element in the art of RFdesign.

Performance metrics such as low insertion loss, high linearity, highisolation and power handling are critical in RF switch design. However,the RF switch performance is limited by substrate interactions with thecircuits which degrade all the key performance metrics as well asnon-linearities in the circuits themselves. Switching to an SOIsubstrate has been shown to improve the loss in the substrate althoughadditional treatment of the silicon substrate is necessary to improvelinearity. This has been achieved using an implant step in the processwhich prevents charge build-up at the silicon surface beneath the oxide.However, these steps are still not sufficient to meet the stringentdemands of wireless technologies and further techniques are required toserve the broader wireless market segment.

A publication by Botula A et al entitled ‘A Thin Film SOI 190 nm CMOS RFSwitch Technology, IEEE, Piscataway, N.J., USA, 19 Jan. 2009,XP031415569 discloses a thin film SOI technology developed for RF switchapplications and provides a buffer to protect the circuits from the RFsignals. Other publications include WO2008/057524 assigned to SkyworksSolutions Inc., US2003/090313, Burgener et al, and US 2005/176184,Okihara Masao. However, none of the RF designs disclosed in thesepublications deal with the problem of unintended interactions of the RFand/or DC (or control) signals which are present, by injection of RFsignals into the DC and low frequency analog blocks or by directcoupling of the wires on an IC chip. If a portion of the RF signalbecomes inadvertently superimposed on the DC bias signals it degradesthe linearity of the circuits making them unsuitable for wirelessapplications that demand low harmonic distortion.

One solution to the substrate interaction problem involves using analternative substrate material such as sapphire. However, thissignificantly increases manufacturing costs and may not completely solvethe interaction of RF and DC signals. Another solution involves the useof III-V technologies such as GaAs but this also increases costs anddecreases integration levels.

There is therefore a need to provide a RF switch system, method andarchitecture to overcome the above mentioned problems.

SUMMARY OF THE INVENTION

According to the present invention there is provided, as set out in theappended claims, a Radio Frequency (RF) system and method comprising:

-   -   a RF switch comprising a plurality of transistor switching        elements implemented on Silicon on Insulator (SOI) for switching        at least one or more RF signals;    -   said SOI comprises a bulk substrate region and a buried oxide        region;    -   characterised by:    -   at least one filter adapted to isolate the RF signal from the        substrate and/or other high frequency signals or control signals        present in the RF system.

The advantage of the invention is that the filter eliminates anypossibility of coupling to neighbouring bias lines or the substratewhich will result in superior linearity in the RF switch and reduce anyRF swing that is associated with prior art RF switches. The systemprovides flexibility in how bias lines are routed on an IC chip and cansave die area on larger switches since isolation between the bias linescan still be achieved without taking precautions to space them far apartor shield them from one another.

In one embodiment there is provided a coupling capacitor adapted tocooperate with said at least one filter to improve linearity of thetransistor switch elements.

In one embodiment the at least one filter is coupled to a gate bias lineof at least one of said transistor switching elements.

In one embodiment there is provided a trench implant layer adapted withmeans for capturing free carriers at the interface between the bulksubstrate region and buried oxide region.

In one embodiment the at least one filter is coupled to a gate bias lineand body bias line of at least one of said transistor switchingelements.

In one embodiment both the gate and body bias signals are connected to afilter which is connected to a bias resistor and connected to the sourceor drain of the switch transistor through a capacitor. This has theeffect of filtering the rf signal from the control line outside the biasresistor preventing coupling to the substrate and neighbouring signalswhile allowing the rf signal to couple locally to the gate and body ofthe switch transistor to improve it's linearity.

In one embodiment the filters are located in close proximity to thenegative voltage generator and digital control block to preventinjection of the RF signal from the transistor bias signals into thesecircuits causing non-linearities.

In one embodiment the filters are distributed across the RF system alongwith the switches to the DC supplies so that filtering occurs directlyat the switch arm which eliminates the need to bus control signals withRF power superimposed across the RF system preventing injection of theRF power into the substrate as well as preventing coupling betweencontrol lines which will improve linearity.

In another embodiment partial filtering occurs locally at the at leastone transistor and further filtering occurs at the digital block andnegative voltage generator circuit blocks.

In another embodiment the width of the first and last transistor in theswitch arm is reduced to fit the coupling capacitors into the sameswitch area without the coupling capacitors.

In a further embodiment the invention utilizes a trench implant layer inthe semiconductor substrate that alleviates the problem of substratecharge build-up. In one embodiment the trench layer comprises means forcapturing free carriers at the bulk substrate/buried oxide interface.The trench layer is typically implanted during the manufacturingprocess. This implanted trench layer captures the free carriers at thebulk substrate/buried oxide interface and so stops the charge build-upthat creates the weak inversion layer. This improves the insertion lossperformance of the RF switch and also reduces the generation of harmonicproducts.

In one embodiment the trench implant layer substantially surrounds atleast one of the transistor switching elements.

In one embodiment at least one transistor comprises a connecteddrain-source resistor, Rds, to improve the linearity performance of theRF system. The use of the trench implant in RF switches yields improvedswitch performance in terms of insertion loss and linearity. The RFswitch section consists of transistors that utilise drain-sourceresistors. The drain-source resistors improve the performance of the RFswitch by increasing the linearity of the circuit.

In one embodiment the plurality of transistors are arranged in a stackedconfiguration.

In one embodiment at least one transistor element comprises a metalcontact layer between source and drain regions using vias.

In one embodiment the metal interconnects to connect the drain of afirst transistor to the source of the next transistor arranged inseries.

In one embodiment a trench layer is positioned underneath the metalcontact layers and/or metal interconnects tracks in the RF system.

In one embodiment said system comprises a digital control block and anegative voltage generator.

In one embodiment a filter comprises means to protect a digital controlblock and the negative voltage generator from high power high frequencysignals present in the RF system.

In one embodiment the negative voltage generator comprises an oscillatorcircuit, a clock generator circuit and a charge pump.

In one embodiment the digital logic block comprises means to control theon-off state of the transistor elements of the RF system.

In a further embodiment of the present invention there is provided aRadio Frequency (RF) system comprising:

-   -   a RF switch comprising a plurality of transistor switching        elements implemented on a layer of Silicon on Insulator (SOI);    -   a digital control block; and    -   a negative voltage generator,    -   wherein the layer of SOI comprises a trench implant layer to        prevent unwanted substrate charge build-up.

In a further embodiment of the invention there is provided a method ofmanufacturing an RF system comprising the steps of:

-   -   arranging a plurality of transistor switching elements        implemented on Silicon on Insulator (SOI), said SOI comprises a        bulk substrate region and a buried oxide region;    -   positioning at least one filter adapted to isolate a RF signal        from the substrate and/or other high frequency signals or        control signals present in the RF system.

In a further embodiment there is provided a system comprising:

-   -   a switch comprising a plurality of transistor switching elements        implemented on a layer of Silicon on Insulator (SOI);    -   said SOI layer comprises a bulk substrate region and a buried        oxide region;    -   wherein a trench implant layer is provided with means for        capturing free carriers at the interface between the bulk        substrate region and buried oxide region.

In another embodiment there is provided a system comprising:

-   -   a switch comprising a plurality of transistor switching elements        implemented on a layer of Silicon on Insulator (SOI);    -   a digital control block; and    -   a negative voltage generator,    -   wherein at least one transistor comprises a connected        drain-source resistor, Rds, to improve the linearity performance        of the system.

In another embodiment there is provided a Radio Frequency (RF) systemcomprising:

-   -   a RF switch comprising a plurality of transistor switching        elements implemented on Silicon on Insulator (SOI) for switching        at least one or more RF signals;    -   said SOI comprises a bulk substrate region and a buried oxide        region;    -   a digital control block;    -   a negative voltage generator characterised by:    -   a trench implant layer adapted with means for capturing free        carriers at the interface between the bulk substrate region and        buried oxide region,    -   at least one filter adapted to isolate the RF signal from the        substrate, the digital control block and/or negative voltage        generator.

In another embodiment there is provided a RF switch comprising aplurality of transistor switching elements implemented on asemiconductor substrate for switching at least one or more RF signals;characterised by at least one filter adapted to isolate the RF signalfrom the substrate and/or other high frequency signals or controlsignals present in the RF system. The semiconductor substrate can be anytype of material, but preferably comprises SOI material.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from the followingdescription of an embodiment thereof, given by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 illustrates a cross-sectional diagram of a typical SOI process;

FIG. 2 illustrates in block diagram of the main elements of an RF switchsystem;

FIG. 3 illustrates an implementation of an RF switch containing twoseries and two shunt arms;

FIG. 4 illustrates a cross-sectional diagram of a single NMOS transistorimplemented in a standard SOI process;

FIG. 5 illustrates the combination of a filter, a rf bias resistor andswitch or shunt transistor, according to a preferred embodiment of theinvention;

FIG. 6 illustrates a filter and bias resistor on both the gate and bodyof the switch or shunt transistor, according to another embodiment ofthe invention;

FIG. 7 illustrates a filter with a bias resistor and a couplingcapacitor on the gate to drain, gate to source, body to drain and bodyto source terminals;

FIG. 8 shows a filter blocking RF signals from reaching the digitallogic blocks and the positive and negative power supplies;

FIG. 9 illustrates an embodiment of a centralized filter bank accordingto another aspect of the invention;

FIG. 10 illustrates the concept of a local filter per switch accordingto one embodiment of the invention;

FIG. 11 illustrates a cross-sectional diagram of the charge build up inthe substrate, occurring at the interface between the bulk substrate andthe buried oxide;

FIG. 12 illustrates a cross-sectional diagram of the trench implantsthat are used to remove the charge build-up in the substrate of an RFsystem according to one aspect of the invention;

FIG. 13 shows a circuit level implementation of a switch arm where anumber of transistors are connected together in series and parallel;

FIG. 14 shows the same transistor level implementation of a switch armas in FIG. 13 except in this case the location of the trench layerimplants is shown;

FIG. 15 shows a layout view of a typical switch arm;

FIG. 16 illustrates a cross-sectional diagram of the switchimplementation of FIG. 15;

FIG. 17 illustrates a block diagram of the principle elements of thenegative voltage generator block; and

FIG. 18 illustrates a block diagram of the principle elements of thedigital control block.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section of the principle elements of atypical SOI process. The SOI process consists of a thin silicon activearea above an insulating layer. This insulating layer sits on top of asilicon substrate. A typical material for the insulating layer issilicon dioxide. In general SOI technologies consist of a bulk substrate1, a buried oxide layer 2 and another thin active silicon layer 3. Thebulk substrate 1 is generally a high resistivity substrate. The bulksubstrate 1 can be either P-type or N-Type. A typical thickness for thebulk substrate is 250 μm. The buried oxide layer 2 is an insulatorlayer, typically silicon dioxide. A typical thickness of the buriedoxide layer 2 is 1 um. The silicon layer 3 above the buried oxide is athin layer of the order of 0.2 μm. FIG. 2 shows a block diagram of theprinciple elements of an RF switch system. A switch system consists ofan RF switch section 10, a digital logic section 11 and a negativevoltage generator section 12. The RF switch 10 shown is a single poletwo throw (SP2T) switch. The switch has four switching elements. Eachswitching element can be turned on or off. Ideally, when the switch 10is on it behaves as a short circuit and when it is off it behaves as anopen circuit. The on/off state of each switching element is controlledby DC voltages from the digital logic control block. The logic inputsare used to control what control voltage is applied to each switchingelement. To meet required performance levels in RF switch design anegative voltage is needed to control the off state of the switchingelement. This negative voltage is supplied by the negative voltagegenerator 12. The negative voltage generator 12 supplies the digitalcontrol block with a constant negative DC voltage. This negative DCvoltage, along with the positive supply voltage, is then used by thedigital control block 11 to control the switching elements.

A typical example of an RF switch section is illustrated in FIG. 3. Thisfigure shows a single pole two throw switch. It contains two series arms20, 21 and two shunt arms 22 23. The series arms 20, 21 are used tocontrol the flow of RF signals between different points of the RFsystem. The shunt devices 22, 23 are used to improve the RF performanceof the circuit by increasing the isolation. The single pole two throwswitch functions as follows: when series arm 20 is on, series arm 21 isoff, shunt arm 22 is off and shunt arm 23 is on. The on/off state ofeach arm is controlled by positive and negative voltages from thedigital control block. The positive and negative control voltages areapplied to the gates of the transistors of each arm through largeresistors (Rg). One important aspect of the present invention is theprovision of drain-source resistors, Rds, 24 that are used on eachtransistor in the switch circuit. These Rds resistors 24 are used toimprove the linearity performance of the switch circuit.

As shown in FIG. 3 each switching element of the switch circuit consistsof five transistors connected is series. This series connection oftransistors is known as “stacking”. Multiple transistors are stacked inthis manner to design a switch that can operate under high voltageconditions. This idea behind using stacked transistors is to replace thesingle transistors in the switch by a series of two or more transistors.As a result, if m transistors are used in the stack, then the voltageseen across the source and drain of each of the transistors will bedistributed evenly and will be reduced by a factor m.

However, the system can contain any number of transistors withoutdeviating from the scope of this invention. An arm is turned on by theapplication of a positive voltage to the gate resistors, Rg, of thetransistors of the particular arm. In the case of series arm 20application of a positive voltage creates a low resistance path betweennode 1 and node 2. The lower the resistance of this path the less RFpower is lost when a signal travels from node 1 to node 2. The amount ofsignal power lost between node 1 and node 2 is known as the insertionloss of the switch.

As shown in FIG. 4 transistors are made in the thin active layer of theprocess. The transistor consists of drain, source and gate regions, gatedielectric and a channel region. The source and drain regions arecreated in this thin silicon layer using a suitable heavy dopant. Thechannel region is the area of active silicon between the source anddrain. The gate dielectric is formed over the channel region. Isolationimplants can also be implanted in areas of unused active silicon.Contact to the source and drain regions can be made using vias thatconnect to the processes' metal layers.

FIG. 5 shows the placement of a filter 60 before a number of biasresistors 61 on the gate of a number of transistors 62 of one of thearms shown in FIG. 3, according to a preferred embodiment of theinvention. The bias resistor 61 is typically a large value of ˜50-100kOhms but is limited in how large it can be by the switching timerequirements of the switch. As a result of this limitation some RF powerwill leak through the resistors to the gate bias line. This RF power cancouple to the substrate from the gate bias line or couple to anotherbias line on the chip resulting in non-linearities in the switchbehaviour. The filter 60 attenuates this RF power/voltage to isolate theRF signal from the substrate and/or other high frequency signals orcontrol signals present in the RF system. FIG. 6 is similar to FIG. 5except there is a filter placed on the body contact as well as the gate.The additional body contact improves the transistor linearity but thisbenefit can be lost if a filter is not present.

FIG. 7 shows the addition of a coupling capacitor 64 to the transistors62 at each end of a switch arm. FIG. 7 illustrates the filter with thebias resistor and the coupling capacitor on the gate to drain, gate tosource, body to drain and body to source terminals. The capacitor 64will feed more of the RF voltage to the transistor gate and body. Thisis advantageous for the transistor since the change in bias withincoming RF power will reduce the voltage differentials on thetransistor and help it's response to be more linear. However, withoutthe presence of the filter 60 more RF power would also be coupled to thebias line adding non-linearities elsewhere in the DC and analog circuitsor through coupling to the substrate through the bias lines.

FIG. 8 is similar to FIG. 7 but shows how the filters isolate thedigital and low frequency analog circuits 65 and 66 from the RF powerwhich will improve linearity. FIG. 8 shows the filter 60 blocking RFsignals from reaching the digital logic blocks 65 and the positive andnegative power supplies 66.

FIG. 9 shows one possible implementation of the filters 60 which arelocated together and in close proximity to the digital and low frequencyanalog circuits 65. FIG. 9 illustrates how bias lines 70 run around thechip and can create coupling according to one embodiment. The bias lines70 are designed to be relatively long can run for approximately amillimetre in length around the chip. Without the presence of an RFfilter 70 significant rf voltage would be present on the bias lines andwould couple to other bias lines or to the substrate which would causenon linearities in the switch characteristics. The design of the filter60 significantly reduces the voltage swings on the bias lines 70.

FIG. 10 shows another implementation where filters 60 a, 60 b, 60 c and60 d (and associated circuits) are placed local to each switch. FIG. 10illustrates how the local filter architecture removes the long biaslines, according to one embodiment. FIG. 10 shows how local filters areused to reduce any rf voltage on the bias lines but more importantlydramatically reduce the length of the bias lines themselves thuseliminating any possibility of coupling to neighbouring bias lines orthe substrate which will result in superior linearity in the switch. Thelocal filter architecture eliminates the long bias lines on the IC chipeliminating any possibility of rf coupling to each other or to thesubstrate.

FIG. 11 shows the region where charge build-up can occur in an SOIprocess indicated generally by the reference numeral 30. Free carriersin the bulk substrate form a weak inversion layer 30 at the interfacebetween the bulk substrate 1 and the buried oxide layer 2. This weakinversion layer 30 has adverse affects on the RF performance of a switchcircuit. It results in a varying parasitic capacitance over the RF cyclethat creates unwanted harmonic products and increases insertion loss ofthe switch circuit. This seriously impacts the usefulness of SOI as aprocess for the design of RF switches. However, an important aspect ofthe invention is that the effects of this inversion layer can be reducedby using a trench implant layer 31. This is illustrated in FIG. 12. Thetrench layer 31 is typically implanted during the manufacturing processhas means for capturing the free carriers at the bulk substrate/buriedoxide interface and so stops the charge build-up that creates the weakinversion layer 30. This improves the insertion loss performance of theRF switch and also reduces the generation of harmonic products. Thetrench implant layer process was developed to reduce harmonic distortioncaused by substrate-circuit interactions. The trench is built after thetransistors have been formed. This is achieved by etching down to thesubstrate where active devices are not present and implanting thesubstrate to create damage which will inhibit the formation of mobileelectrons at the substrate interface.

As shown in FIG. 3 typical RF switch applications utilise a number oftransistors connected in series. This is done so that the switch circuitcan operate under high voltage conditions. Typical switch designs alsoutilise a number of transistors connected in parallel. FIG. 13 shows atypical implementation of a single arm of a switch circuit. Asillustrated there are M transistors connected, or stacked, in series. Mcan be any number depending on the particular design requirements. Eachseries transistor in the stack of M transistors can themselves be madeup of N transistors connected in parallel. N can be any number dependingon the particular design requirements. However the weak inversion layer30 has adverse affects on the RF performance of a transistors, resultingin a varying parasitic capacitance over the RF cycle that createsunwanted harmonic products and increases insertion loss of the switchcircuit as outlined above.

FIG. 14 shows the same implementation as FIG. 15 except in this case thetrench implant layer 31 is utilized. As can be seen the trench layer 31surrounds each of the parallel transistors from 1 to N and each of thestacked cells from 1 to M. The trench layer improves the insertion lossperformance of the RF switch and also reduces the generation of harmonicproducts.

FIG. 15 shows a layout view of three of the M series transistors and twoof the N parallel transistors of FIG. 4. As can be seen each transistoris completely surrounded by the trench implant layer 31. FIG. 16 shows across-section of FIG. 15. Metal interconnects are used to connect thedrain of each transistor to the source of the next series transistor. Atrench implant is used underneath each of these metal interconnects andbetween each transistor in the stack. Those skilled in the basic art ofRF switch layout will see that many similar layout implementations arepossible while not deviating significantly from the invention describedhere.

As shown in FIG. 2 as well as the RF switch itself is a complete switchsystem comprised of a digital block and negative voltage generator block12. FIG. 17 shows a block diagram of the principle elements of anegative voltage generator. The negative voltage generator 12 containsan oscillator circuit 40, a clock generator circuit 41 and a charge pump42. As stated already the arms of the RF switch need to be turned on andoff at various times. For many applications a negative voltage isrequired to turn the transistor arm off. This is due to high powernature of certain applications and also to the stringent linearityrequirements that must be met.

FIG. 18 shows a block diagram of the principle elements of the digitalcontrol block 11. The digital logic block in its simplest form iscomprised of a decoder circuit 50, level shifter 51 and a filter 52,which functions in the same way as the filter 60 described above. Thelevel shifter 51 takes the negative voltage generated in the negativevoltage generator as one of its inputs. The filter 52 prevents unwantedhigh power RF signals from the RF switch from damaging the digital logicblocks and the negative voltage generator block. The function of thedigital logic block is to control the on-off state of the transistorarms of the RF switch. The RF transistors operate under high powerconditions. Under certain operating conditions the level shifter can beexposed to these high power RF signals. The filter 52 is positionedbetween the level shifter outputs and the gate resistors of the RFtransistors. The filter 52 functions by blocking any RF signals fromreaching the level shifter and so protect the circuitry from damage.

It will be appreciated that in the context of the present invention SOIis a preferred substrate to bring the invention into effect, however itis envisaged any type of semiconductor substrate can be used that issuitable for making a RF switching system as hereinbefore described withrespect to the description and/or figures.

In the specification the terms “comprise, comprises, comprised andcomprising” or any variation thereof and the terms include, includes,included and including” or any variation thereof are considered to betotally interchangeable and they should all be afforded the widestpossible interpretation and vice versa.

The invention is not limited to the embodiments hereinbefore describedbut may be varied in both construction and detail.

1. A Radio Frequency (RF) system comprising: a RF switch comprising aplurality of transistor switching elements implemented on Silicon onInsulator (SOI) substrate for switching at least one or more RF signals;said SOI comprises a bulk substrate region and a buried oxide region;characterised by: at least one filter adapted to isolate the RF signalfrom the substrate and/or other high frequency signals or controlsignals present in the RF system.
 2. The RF system of claim 1 wherein acoupling capacitor is adapted to cooperate with said at least one filterto improve linearity of the transistor switch elements.
 3. The RF systemas claimed in claim 1 comprising a trench implant layer adapted withmeans for capturing free carriers at the interface between the bulksubstrate region and buried oxide region.
 4. The RF system as claimed inclaim 1 wherein the at least one filter is coupled to a gate bias lineof at least one of said transistor switching elements.
 5. The RF systemas claimed in claim 1 wherein the at least one filter is coupled to agate bias line and body bias line of at least one of said transistorswitching elements.
 6. The RF system as claimed in claim 1 wherein gateand body bias signals are connected to the at least one filter which isconnected to a bias resistor and connected to the source or drain of theswitch transistor through a capacitor.
 7. The RF system as claimed inclaim 1 wherein partial filtering occurs locally at the at least onetransistor and further filtering occurs at the digital block andnegative voltage generator circuit blocks.
 8. The RF system as claimedin claim 1 wherein the width of the first and last transistor in theswitch is reduced to fit the coupling capacitors into the same switcharea without the coupling capacitors.
 9. The RF system as claimed inclaim 1 wherein the trench implant layer substantially surrounds atleast one of the transistor switching elements.
 10. The RF system asclaimed in claim 1 wherein at least one transistor comprises a connecteddrain-source resistor, Rds, to improve the linearity performance of theRF system.
 11. The RF system of claim 1 wherein the plurality oftransistors are arranged in a stacked configuration.
 12. The RF systemof claim 1 comprising metal interconnects to connect the drain of afirst transistor to the source of the next transistor arranged inseries.
 13. The RF system of claim 1 comprising metal interconnects toconnect the drain of a first transistor to the source of the nexttransistor arranged in series wherein a trench layer is positionedunderneath the metal contact layers and/or metal interconnects tracks inthe RF system.
 14. The RF system of claim 1 wherein said systemcomprises a digital control block and a negative voltage generator. 15.The RF system as claimed in claim 1 wherein said system comprises adigital control block and a negative voltage generator and the at leastone filter comprises means to isolate the digital control block and thenegative voltage generator from high power high frequency signalspresent in the RF system.
 16. The RF system as claimed in claim 1wherein said system comprises a digital control block and a negativevoltage generator, the negative voltage generator comprises anoscillator circuit, a clock generator circuit and a charge pump.
 17. TheRF system as claimed in claim 1 wherein said system comprises a digitalcontrol block and a negative voltage generator, the digital logic blockcomprises means to control the on-off state of the transistor elementsof the RF system.
 18. A method of manufacturing an RF system comprisingthe steps of: arranging a plurality of transistor switching elementsimplemented on Silicon on Insulator (SOI), said SOI comprises a bulksubstrate region and a buried oxide region; and positioning at least onefilter adapted to isolate a RF signal from the substrate and/or otherhigh frequency signals or control signals present in the RF system. 19.The method of claim 18 comprising the step of implanting a trenchimplant layer between said substrate and buried oxide region forcapturing free carriers at the interface between the bulk substrateregion and buried oxide region.
 20. The method of claim 18 comprisingthe step of implanting a trench implant layer wherein the trench implantlayer substantially surrounds at least one of the transistor switchingelements.